1. Technical Field
The present invention relates to coding and decoding of an audio signal, and more specifically to a wideband audio signal coding and decoding apparatus and method capable of coding and decoding a wideband audio signal while maintaining a low bit rate.
2. Related Art
A voice coder, usually used for mobile communications services or VoIP (Voice over Internet Protocol) services, processes a narrowband signal whose bandwidth is less than 4 kHz.
For example, a VoIP voice coder processes a narrowband signal using a voice coder such as ITU-T G.729, ITU-T G.723.1, ITU-T G.728, or iLBC (Internet Low Bit-rate Codec), and then transmits the bitstream of the processed narrowband signal over an IP network.
The above-mentioned VoIP voice coder is appropriate for coding a narrowband voice signal, but not for a wideband signal that requires higher quality than a voice signal (for example, a music signal used for ring back tone services).
That is, the above-mentioned VoIP voice coder compresses an input signal into a signal having a low bit rate (for example, 5.3 to 15 kbit/s) under the assumption that the input signal has a bandwidth of substantially less than 3.4 kHz.
However, a high quality audio signal generally has a bandwidth of more than 4 kHz, and a coder should be able to process a wideband signal whose bandwidth is substantially more than 7 kHz in order to improve quality of an audio signal.
Moreover, a signal, which has been coded with a high bit rate, increases the packet size, and therefore, is prone to cause a packet loss in a transmission environment such as IP based networks, thus leading to lowering quality of the decoded audio. For example, a G.722 standard wideband coder, used for VoIP services, may code a 7 kHz wideband signal with a bit rate of 48, 56, or 64 kbit/s. However, the G.722 coder may give rise to quality degradation due to the high bit rate in a transmission environment such as IP based networks.
A standard for audio coders, such as MP3 (MPEG-1/2 Layer III) or AAC (Advanced Audio Coding) in by MPEG (Moving Picture Experts Group), etc., has been developed as a method for improving communication quality of audio signals. However, the above-mentioned audio coders have a disadvantage of being inappropriate for use in current mobile communications and VoIP service environments due to a high bit rate.
A wideband coder having a variable bit rate of a scalable or embedded method has been suggested to provide improved communication quality in an environment that requires a low bit rate such as mobile communications and IP network environments as a method for supplementing the disadvantage (A. Kataoka, S. Kurihara, S. Sasaki, and S. Hayashi, “A 16-kbit/s wideband speech codec scalable with G.729,” Proc. Eurospeech, pp. 1491-1494, September 1997).
FIG. 1 is a conceptual view illustrating an operation principle of a wideband voice coder having a variable bit rate according to the prior art.
Referring to FIG. 1, a conventional embedded-type wideband voice coder having a variable bit rate includes a core coder 11, an enhancement layer 12, and a packet generating unit 13. The core coder 11 codes narrowband signals out of inputted audio signals. The enhancement layer 12 transmits additional bits depending on a network environment. The packet generating unit 13 packetizes signals outputted from the core coder 11 and the enhancement layer 12 to output a bit stream.
That is, the conventional embedded-type wideband coder codes narrow signals out of inputted audio signals with a low bit rate in the core coder 11. And, the conventional embedded-type wideband coder transmits only the signals coded in the core coder 11 to prevent the transmission loss if there are lots of traffics in a network, and transmits additional bits in the enhancement layer 12 to improve quality of audio signals if there are small traffics in the network.
However, since the enhancement layer 12 has been configured independently from the core coder 11 to increase bandwidth without considering the core coder 11 in the wideband voice coder having a variable bit rate, shown in FIG. 1, it is difficult to implement the enhancement layer 12 to have a low bit rate. Also, the enhancement layer 12 has been configured to process the same amount of information as that of the core coder 11 to substantially improve communication quality, which may increase the entire amount of information, thus causing the conventional coder to be inappropriate for transmission of wideband audio signals in mobile communications or IP based network environments.
A first aspect of the present invention provides a wideband audio signal coding/decoding device capable of coding wideband audio signal while maintaining a low bit rate.
A second aspect of the present invention provides a wideband audio signal coding/decoding method capable of coding a wideband audio signal while maintaining a low bit rate.